The prompt fission neutron multiplicity and spectra of actinides are nuclear data of crucial importance. Based on experimental fission yield and total kinetic energy data, new calculations for the prompt neutron multiplicity and spectra for 235U(n,f) in the incident neutron energy range up to 50 MeV and for 252Cf(SF) have been performed.
For 235U(n,f) a new fission cross section calculation was necessary up to 50 MeV to be able to deduce the particle fission cross sections above the first fission chance and their ratios, quantities entering the multiplicity and spectrum modelling. For 252Cf(SF), the multi-modality of the fission process was taken into account. Additionally, a more realistic fission fragment residual temperature distribution as well as an anisotrophy of the prompt neutron emission led to improved agreement between the calculation and experimental results.
Also the range of fission fragment pairs entering in the multiplicity and spectrum model was extended over the entire experimental fission fragment mass range. This led to an improved version of the Los Alamos model and especially to an improved determination of the input model parameters. The results of the calculations will be presented.